If a minor commits libel, are their parents or guardian liable for the tort?
No. I am not aware of any jurisdiction in the United States where a parent is liable for defamation torts committed by their child (assuming, of course, that the child is not simply republishing defamation which was first uttered by the parent).
The general rule is that a parent is not liable for a child's tort, subject to exceptions (e.g. the family car rule, negligent supervision of a child, etc.). No exception applies to defamation by a minor.